1. Field of the Invention
The invention relates to charged particle guns, and more particularly to field electron and ionization sources for producing high current, medium energy electron and ion beams with electrostatic lens systems for focusing the beams into small spot areas on a target, particularly for use in lithographic systems for semiconductor manufacturing.
2. Description of the Prior Art
The low-aberration three-electrode electrostatic lens is the most recent development evolving from the well known einzel lens. Beginning with the einzel lens, R. Seeliger in Optik 4, 258 (1948), "Ein neues Verfahren Zur Bestimmung des e,uml/O/ ffnungsfehlers von Electronenlinsen" ("A new method for determination of aperture aberrations of electron lenses") noted that a physically asymmetric inner (focus) electrode lowers spherical aberration.
Reed and Imhoff (J. Sci, Instruments 1, 859, 1968) electrically decoupled the three electrodes of the einzel lens to produce a zoom lens, which allows the use of a variable energy and variable magnification beam to impinge on the target. Equivalently, a variable gun focus is obtained without having to move the object along the axis.
Veneklasen and Siegel (J. Appl. Phys. 43, 4989, 1972) developed a relatively complex einzel lens "preaccelerator" electrode geometry for field emission transmission electron microscopy applications. They noted the use of a high magnification operation can reduce gun aberrations, although the principle is not consistently applied as it is assumed there is a distinction between electrostatic and magnetic lenses. Actually the principle holds in either case, and was first demonstrated experimentally in an election optics application by K.-J. Hanszen in Optik 15, 304 (1958), and is a rediscovery of a well known classical optics result for reducing spherical aberration, see A. Septier, Ad. Opt. Electr. Microscopy issue 1, p. 221.
Kuroda et al. (J. Appl. Phys. 45, 2336, 1974) adopted Reed and Imhoff's results and applied it to develop a lens for field emission scanning electron microscopy. This design reduced the complexity of Veneklasen's preaccelerator while maintaining its functionality.
G.H.N. Riddle in J.Vac.Sci.Technol. 15, 857 (1978) investigated a set of sixteen three-electrode lens geometries for field emission guns. He found that a physically asymmetric focus electrode geometry lowers the spherical aberrations and confirmed Seeliger's original finding that his einzel lens geometry had a small spherical aberration. Riddle also found this design has the lowest chromatic aberration of all the lenses studied. These aberrations are well known to workers in the field. Chromatic aberration contribution to beam diameter is d.sub.c=C.sub.c .alpha. where C.sub.c is the chromatic aberration coefficient, .alpha. is beam semiangle, .DELTA.E is the measure of the beam energy distribution, and E is the mean value of the beam energy. The spherical aberration contribution to the beam size is d.sub.s =1/2 C.sub.s .alpha..sup.3 with C.sub.s the spherical aberration coefficient. Otherwise stated, spherical aberration increases beam size due to variation in focal length with off-axis distance of the beam. Chromatic aberration increases beam size due to the velocity dispersion of the beam.
Orloff and Swanson (U.S. Pat. No. 4,426,582 incorporated herein by reference) used Riddle's low chromatic aberration gun lens as a focusing element for an ion beam microprobe. They also verified the lens' aberration properties using Munro's electron optics computer programs and showed this lens had better performance than either of the two-electrode lenses of Crewe or Munro. Although the Orloff and Swanson patent disclosure is of the use of the three element electrostatic lens, its primary emphasis is on its application in conjunction with a liquid metal ion source. Although the lens is emphasized as having low aberration properties, the reduction in object distance to decrease aberrations is not considered critical by them. Also in their application spherical aberration was not considered the dominant factor and thus they did not consider other measures to reduce this factor contributing to the beam crossover size at the gun image plane. The system of Orloff and Swanson is not therefore suitable for use in a lithography system requiring large currents and submicron beams because of its large aberrations.
In the above-referenced Orloff and Swanson patent disclosure and their article in J. Appl. Phys. vol.50 (4), (1979) p.2494-2501, the source to object distance is z.sub.o =15 to 20 mm. FIG. 1 of the present application is from the Orloff and Swanson patent disclosure. Ion gun 1 includes a liquid metal field ionization source 11 and electrostatic optical system 13, a deflection system 21, and a target 9.
Field ionization source 11 includes an emitter support 11A, and an emitter 11B. The tungsten field emitter 11B is welded to a tungsten loop 11A to resistively heat the emitter.
The above described elements are supported within a vacuum chamber 5 on support 3. Target 9 is on support 7. Support 7 extends through the wall of a vacuum chamber 5 to permit alignment of target 9 with the beam emitted from emitter 11B and focused by the electrostatic lens system 13. The ion beam is focused generally along dotted line 19 to target 9.
An extractor ring 15 having an opening 15A therein is supported and aligned with emitter 11B immediately below the tip of emitter 11B. Extractor ring 15 is at the same potential as element 13A of electrostatic optical system 13 and physically attached to element 13A.
Asymmetric electrostatic lens system 13 includes top element 13A, center element 13C, and bottom element 13F.
A first element 13A is a disk-shaped conductor having a circular aperture 13B centered about axis 19, and first element 13A is perpendicular to axis 19. Aperture 13B has a diameter D, which is three millimeters. The thickness of first element 13A is t',equal to one millimeter.
A second element 13C is physically asymmetric, and is spaced a distance S from first element 13A, S being three millimeters physical asymmetry results from an aperture of varying size through second element 13C. The upper portion 13D of second element 13C has a thickness t and an aperture diameter of D, t being three millimeters. The lower portion 13E of second element 13C has a thickness of 14 millimeters and an aperture diameter of D', which is 18 millimeters. The ratio of the aperture diameter D' to the thickness of lower portion 13E therefore is 18 millimeters divided by 14 millimeters, or approximately 1.3.
A third element 13F is parallel to first element 13A, and is spaced S millimeters therefrom. Third element 13F has a thickness t' and an aperture diameter of D. First, second and third elements 13A, 13C and 13F are all axially aligned with respect to axis 19.
The distance between the tip of emitter 11B and the plane of the upper surface of top element 13A is 15 millimeters. The distance between the target spot on target 9 and the plane of the upper surface of upper element 13A is 75 millimeters.
Electrostatic deflection system 21 is disposed below third element 13F in FIG. 1 and is axially aligned with axis 19. A voltage V.sub.E is applied to the emitter.
An initial or first voltage V1 is applied to first element 13A by means of feedthrough 17A. Voltage V1 is also applied to extractor ring 15 in the device of FIG. 1. A control voltage V2 is applied to center element 13C by means of feedthrough 17B.
A third or final voltage V3 is applied to bottom element 13F by means of feedthrough 17C. V1, V2 and V3 are all referenced to the voltage V.sub.E of the ionization source 11. V3 is set at ground potential and the final energy of the beam then be determined by V.sub.E. The extraction voltage is determined by the difference between V.sub.E and V1. 0 Feedthrough 17D applies suitable voltage to the deflection plates of electrostatic deflection system 21. The commercially available Schottky-SEM electron gun, based on the above-described Orloff and Swanson gun and manufactured by FEI Co., uses z.sub.o =20 mm.